Flow processes forcing the development of the scour hole Beerenplaat in the Rhine-Meuse delta

Abstract

The Rhine-Meuse delta is characterized by the presence of many scour holes. Koopmans (2017) showed that these scour holes have very different growth rates, even if they are located within the same river branch. Most likely this is caused by either the heterogeneous subsoil, the presence of hydraulic structures, the geometry of the river or anthropogenic influences. Therefore, Koopmans (2017) advises to individually study scour holes in the delta since the local variations in conditions cause different shapes, sizes and growth rates, making every scour hole unique. One of these is the scour hole Beerenplaat, which is located at the confluence/bifurcation of the Oude Maas and the Spui. It is a large scour hole which recently gained much more attention due to the construction of a pilot nourishment by Rijkswaterstaat.

This thesis investigates the morphological development of the river bottom at the scour hole Beerenplaat and the location of the pilot nourishment, by analyzing measurements taken with Multibeam Echosounder. Due to the lack of knowledge on the composition of the heterogenous subsoil, it is tried to understand the observed morphological development based on the hydraulic conditions. At the scour hole Beerenplaat, the hydraulic conditions are characterized by two types of flow processes. The first type of flow process is caused by the confluence/bifurcation Oude Maas and Spui. With the help of Acoustic Doppler Current Profiler (ADCP) measurements and the output of a Delft3D model, the flow field at the confluence/bifurcation was investigated for four scenarios: peak ebb flow, flow changing from ebb to flood, peak flood flow and flow changing from flood to ebb. These scenarios are each paired with different flow features such as flow accelerations, flow stagnation and additional turbulence. It is expected that the second type of flow processes at the scour hole Beerenplaat are the 3D flow processes, which are flow contraction, a (curved) recirculation zone and the horseshoe vortex. Again, the output of the Delft3D model and the ADCP measurements are used to investigate them. Unfortunately, the limitations of both the measurement and the model and the use of an incorrect geometry of the scour hole Beerenplaat, made it not possible to succeed in this. Based on the actual profile of the scour hole, it is expected that the (curved) recirculation zone develops at the steep slope of the scour hole Beerenplaat during ebb flow.

By computing bed shear stresses and using literature, it was investigated how these flow processes attack the river bed during ebb flow, indicating areas vulnerable to scour. It was found that these areas were no match with the observed morphological development. Due to the incorrect geometry of the scour hole in the model, it was not possible to include the hydraulic forcing during flood flow. Most probably, it should be included to describe the expansion of the scour hole and growth towards the Spui. Despite, this research has given better understanding of the flow processes occurring at the scour hole Beerenplaat are shaped and affected by the tide.